By its very nature, email servers have a public face. Companies offer email services to their employees and customers in order to facilitate collaboration with each other across the Internet. Every company that has a public domain allows SMTP traffic through their firewall in order to accept email messages from other users on the Internet. This high visibility makes an SMTP server potentially more susceptible to attack as apposed to other services. Therefore, you need to make sure that your email server is well protected from outside intruders and diffuse any harmful attacks before they occur.